The recent advancement in electronic technology has led to a remarkable proliferation of portable electronic equipment, such as cellular phones, portable personal computers, personal data assistances (PDAs), and portable game machines. With such a proliferation, there has been an increasing demand for power storage devices, such as secondary batteries capable of being repeatedly charged and discharged, the power storage devices serving as power sources for portable electronic equipment. Among such power storage devices, lithium ion secondary batteries using lithium ions as a mobile carrier have been widely used as power sources for portable electronic equipment, because lithium ion secondary batteries have high electromotive forces and high energy densities and the reduction in size thereof is comparatively easy.
In making portable electronic equipment more widely available, further improvement in performance of the portable electronic equipment has been required, in which there have been important technical problems to be solved, for example, reduction in weight and size, improvement in functionality, and the like. In order to solve these technical problems, it has been required for the batteries serving as power sources, for example, to have an improved energy density. One most possible method for improving the energy density of a battery is a method of using an electrode active material having a high energy density. Under these circumstances, active researches and developments on a novel electrode active material having a high energy density have been conduced, regardless of whether the active material is for a positive electrode or a negative electrode.
For example, there has been examined a utilization as an electrode active material of an organic compound having a property such that the electron transfer occurs associated with a reversible redox reaction. Organic compounds have specific gravities of about 1 g/cm3, and are lighter in weight than inorganic oxides such as lithium cobalt oxide, which have been conventionally used as an electrode active material. However, organic compounds, in particular, organic compounds having low molecular weights, are easy to dissolve in organic solvents, and therefore, it has been impossible to use such organic compounds as an electrode active material for use in the currently widely available lithium secondary batteries.
This is attributable to the fact that a non-aqueous electrolyte prepared by dissolving a supporting salt in a non-aqueous solvent being an organic solvent is a mainstream electrolyte of lithium secondary batteries. If an organic compound is dissolved in the non-aqueous solvent, the electron conductivity between a current collector and the organic compound serving as the electrode active material becomes insufficient, and the reactivity is reduced. Moreover, if the organic compound serving as the electrode active material leaches into the non-aqueous solvent, the concentration of the electrode active material capable of being involved in a redox reaction is reduced, and thus the battery capacity is reduced.
As a solution for this, various proposals have been made, suggesting polymerizing the organic compound serving as the electrode active material, using the electrolyte in the form of solid, and the like. For example, there have been numerous proposals that suggest using as the electrode active material an electrically conductive polymer compound in which the whole molecule is covered with conjugated electron clouds, such as polythiophene, polyaniline, and polypyrrole. However, the problem is that the number of reaction electrons in such a conductive polymer compound is as small as about 0.5. From the fact that the energy density of the electrode active material increases in proportion to the number of reaction electrons, it is clear that such a conductive polymer compound fails to have a sufficient energy density.
The reason why the number of reaction electrons is small is described below by taking polythiophene as an example. Polythiophene has a molecular structure in which thiophene rings are adjacent to one another. It is considered theoretically that per one thiophene ring, a reaction involving an exchange of one electron occurs, that is, a one-electron reaction occurs. However, in polythiophene, which is in an electrically charged state as a result of being involved in a redox reaction, due to the electronic repulsion between the adjacent thiophene rings, the reaction that actually occurs involves an exchange of only about 0.5 electrons. Such electronic repulsion is produced similarly in polyaniline and polypyrrole.
In view of the problems of the conventional conductive polymer compounds, for example, one proposal suggests using a conductive polymer compound prepared by introducing a redox-active quinone functional group into the molecule of polyaniline, as a positive electrode active material for a secondary battery (see, e.g., Patent Document 1). The conductive polymer compound of Patent Document 1 is produced, for example, through polymerization of aminobenzenes having two quinone functional groups at the meta position or the ortho position.
The technique of Patent Document 1 intends to increase the number of reaction electrons by introducing a quinone functional group, which permits two-electron reaction involving an exchange of two electrons, into polyaniline being the conductive polymer compound in which the number of reaction electrons is small. However, in terms of the whole conductive polymer compound of Patent Document 1, the number of reaction electrons is averaged and the actual number of reaction electrons is less than two. In other words, in using a compound having a quinone functional group as the electrode active material in order to improve the energy density of a battery, the technique of Patent Document 1 fails to fully utilize the 2-electron reaction, which is the most advantageous feature of the quinone functional group.
Another proposal suggests a composite electrode for a battery including a quinone compound and a nitrogen-containing polymer compound in combination as an electrode active material (see, e.g., Patent Document 2). The technique of Patent Document 2 utilizes that the quinone compound and the nitrogen-containing polymer compound are bound by intermolecular hydrogen bonding, in order to immobilize the quinone compound on the nitrogen-containing polymer compound and form a composite electrode active material having a high energy density. Further, since the reversibility of reaction of the quinone compound with lithium ions is poor, in order to compensate the poor reversibility, an electrolyte containing protons or having proton conductivity is used, so that only protons are involved in the electron transfer associated with the redox reaction of the composite electrode active material. Here, the quinone compound is exemplified by naphthoquinone, anthraquinone, and the like. The nitrogen-containing polymer compound is exemplified by polyaniline, polypyridine, polypyrimidine, and the like.
However, when an electrode including the composite electrode active material of Patent Document 2 is used in a high-voltage design lithium secondary battery including a counter electrode capable of charging and discharging lithium ions, and an electrolyte being a non-aqueous electrolyte solution, the electric potential difference between the positive electrode and the negative electrode is unlikely to exceed 1.2 V, and therefore, the improvement in the energy density of a battery cannot be reliably achieved. This is probably because the electrode active material is synthesized as a composite material by utilizing hydrogen bonding. Since the current lithium secondary batteries are required to have an electric potential difference of much greater 1.2 V between the positive electrode and the negative electrode, the composite electrode active material of Patent Document 2 is lacking in practicality.
Yet another proposal suggests, as an example of an organic compound synthesized by using 9,10-phenanthrenequinone as a starting compound, a 9,10-bis(N,N-diarylamino)phenanthrene derivative (see, e.g., Patent Document 3). The phenanthrene derivative of Patent Document 3, which is used as an electric charge transporting material of an electrophotographic photosensitive body, is devoid of a ketone group and is a monomer that is soluble in an organic solvent, and therefore, is difficult to use as the electrode active material as it is.
Still another proposal suggests a phenanthrenequinone compound in which one of the two oxo groups in 9,10-phenanthrenequinone is replaced with two phenol groups (see, e.g., Patent Document 4). Patent Document 4 simply discloses that this phenanthrenequinone compound can be used as a raw material of resin materials, resist materials, and the like. This phenanthrenequinone compound is also a monomer that is soluble in an organic solvent.
On the other hand, in view of the fluorescence property of phenanthrenequinone compounds, one report suggests utilizing the phenanthrenequinone compounds as organic electroluminescence materials or chemical sensors (see, e.g., Non-Patent Document 1). This report examines polymer substances synthesized by incorporating phenanthrenequinone compounds and sites such as phenol thereinto, which, however, are also polymer substances that are soluble in an organic solvent.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 10-154512    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-82467    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 6-211757    Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-213634    Non-Patent Document 1: Organic Letters, 2006, Vol. 8, No. 9, 1855-1858